Set Content-Disposition in RandomPage HTTP Helper

Fixes: https://github.com/TheTorProject/ooni-probe/issues/305
Verify the sha 256 sum of the zlib package.
Escape all strings before writing them to the log file.

This fixes: https://github.com/TheTorProject/ooni-probe/issues/302 for the backend
Merge pull request #43 from TheTorProject/fix/issue_g

Set Content-Disposition in RandomPage HTTP Helper
Merge pull request #44 from TheTorProject/fix/issue_d

Verify the sha 256 sum of the zlib package.
Merge pull request #45 from TheTorProject/fix/issue_c

Escape all strings before writing them to the log file.
Set limits on the number of headers that can be sent and their length.